There is growing interest in high-quality projection systems that display three-dimensional (3D) or perceived stereoscopic content in order to offer consumers an enhanced visual experience in large venues. Although a number of entertainment companies have offered stereoscopic content in theaters, theme parks, and other venues, these companies have primarily employed film media for stereoscopic image presentation. To create the stereo image, two sets of films are loaded to two separate projection apparatus, one for each eye. Left- and right-eye images are then simultaneously projected using polarized light. One polarization is used for the image presented to the left eye; light of the orthogonal polarization is then used for the image presented to the right eye. Audience members wear corresponding orthogonally polarized glasses that block one polarized light image for each eye while transmitting the orthogonal polarized light image.
In the ongoing transition of the motion picture industry to digital imaging, some vendors, such as Imax, have continued to utilize a two-projection system to provide a high quality stereo image. More commonly, however, conventional projectors have been modified to enable 3D projection.
The most promising of these conventional projection solutions for multicolor digital cinema projection employ, as image forming devices, one of two basic types of spatial light modulators (SLMs). The first type of spatial light modulator is the Digital Light Processor (DLP) a digital micromirror device (DMD), developed by Texas Instruments, Inc., Dallas, Tex. DLPs have been successfully employed in digital projection systems. DLP devices are described in a number of patents, for example U.S. Pat. Nos. 4,441,791; 5,535,047; 5,600,383 (all to Hornbeck).
The second type of spatial light modulator used for digital projection is the LCD (Liquid Crystal Device). The LCD forms an image as an array of pixels by selectively modulating the polarization state of incident light for each corresponding pixel. LCDs appear to have some advantages as spatial light modulators for high-quality digital cinema projection systems. These advantages include relatively large device size, favorable device yields and the ability to fabricate higher resolution devices, for example 4096×2160 resolution devices available from Sony and JVC Corporations. Among examples of electronic projection apparatus that utilize LCD spatial light modulators are those disclosed in U.S. Pat. No. 5,808,795 (Shimomura et al.) and elsewhere. LCOS (Liquid Crystal On Silicon) devices appear particularly promising for large-scale image projection. However, with LCD components it can be difficult to maintain the high quality demands of digital cinema, particularly with regard to color and contrast, since the high thermal load of high brightness projection affects polarization qualities of these devices.
Conventional methods for forming stereoscopic images from these conventional micro-display (DLP or LCOS) based projectors use either of two primary techniques to distinguish between the left and right eye content. One less common technique, utilized by Dolby Laboratories, for example, uses color space separation, as described in US Patent Application Publication No. 2007/0127121 by Maximus et. al. and elsewhere. Filters are utilized in the white light illumination system to momentarily block out portions of each of the primary colors for a portion of the frame time. For example, for the left eye, the lower wavelength spectrum of Red, Blue, and Green (RGB) is blocked for a period of time. This alternates with blocking the higher wavelength spectrum of Red, Blue, and Green (RGB) for the other eye. The appropriate color adjusted stereo content that is associated with each eye is then presented to each modulator for the eye. The viewer wears a corresponding filter that similarly transmits only one of the two 3-color (RGB) spectral sets.
The second method for forming separate stereoscopic images uses polarized light. In the example embodiment of U.S. Pat. No. 6,793,341 to Svardal et al. and elsewhere, each of two orthogonal polarization states is delivered to a corresponding one of two separate spatial light modulators. Polarized light from both modulators is then projected simultaneously. The viewer wears polarized glasses with polarization transmission axes for left and right eyes orthogonally oriented with respect to each other.
Another approach, commercialized by Real-D, Beverly Hills, Calif., uses a conventional projector modified to modulate alternate polarization states that are rapidly switched from one to the other. This can be done, for example, where a DLP projector has a polarizer placed in the output path of the light. The polarizer is required, since the DLP is not inherently designed to maintain the polarization of the input light, which is generally unpolarized, as the window of the device package depolarizes due to stress induced birefringence. An achromatic polarization switcher, similar to the type described in US application 2006/0291053 by Robinson et al. could be disposed after the polarizer. A switcher of this type alternately rotates polarized light between two orthogonal polarization states, such as linear polarization states, to allow the presentation of two distinct images, one to each eye, while the user views with polarized glasses.
Regardless of whether stereoscopic or monoscopic images are formed, digital projection systems have recently incorporated solid-state light sources, particularly LEDs and lasers, and arrays of these sources. These solid-state light sources offer a number of advantages over earlier lamp-based illumination sources used for color projection. Among such advantages are component life, spectral characteristics, brightness, and overall efficiency. For example, when compared against arc lamp and other solutions using a single white light source, solid-state sources expand the available color gamut for projection.
One problem raised by the use of solid-state light sources relates to achieving a suitable balance of the light output from each color channel. Driver circuitry for solid-state light sources can be factory-calibrated to obtain a target white point or color balance from a projector. However, factors such as component aging and drift can degrade the initial color adjustment so that the color balance is no longer acceptable. This problem is even more pronounced for stereoscopic imaging. An image formed for the left eye of the viewer should closely match the corresponding image formed for the right eye of the viewer in terms of overall brightness and color balance. Failure to achieve a compatible intensity of color channels for left- and right-eye images can render the projected stereoscopic images as unappealing or, at worst, as visually disturbing to the viewer.
Thus, there is a need to provide a greater measure of control over light intensity output by light projection systems, for monoscopic as well as for stereoscopic projection.